In rotating type working machines such as a shovel or a crane, as described in the patent literature 1, electric motor driving systems for controlling a rotating direction and a rotating speed by using an electric motor as a rotating driving source and by changing rotation directions and speeds of the electric motor have been known.
According to the electric motor driving systems, energy efficiency can be improved as compared to hydraulic motor driving systems.
Further, in the electric motor driving systems, as described in the patent literature 2, a technique in which a parking brake is operated in a state that rotating operation is stopped, and a rotating body is stopped and held has been known.
In the working machines equipped with a parking brake, external force (hereinafter, referred to as rotating external force) can be generated to the rotating body in a direction of rotating by excavation reaction force due to, for example, excavation by a working device (boom, arm, bucket) of a shovel.
In this case, if the parking brake is working, excessive force can act on the parking brake and a rotating driving part (rotating electric motor, deceleration mechanism) by the rotating external force and the parking brake and the rotating driving part can be damaged.
On the other hand, for a device which employs a hydraulic motor driving system which uses a hydraulic motor as a driving source, as described in the patent literature 3, a technique of releasing a parking brake when a working device is operated has been proposed. This thought can be applied to the devices of electric motor driving system, it is possible to protect a parking brake and a rotating driving part by letting rotating external force loose by releasing the parking brake.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-93210
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-11897
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2003-184808